1. Field of the Invention
The present invention relates to a device and a method for activating an electric machine.
2. Description of the Related Art
Generators for converting mechanical energy into electrical energy in motor vehicles are known. Claw pole generators are usually used for this purpose. According to the related art, these are usually equipped with electrical excitation. Since claw pole generators generate polyphase current, for example, three-phase or five-phase current, rectifying is required for the typical DC voltage vehicle electrical systems of motor vehicles. The related art includes rectifiers based on semiconductor diodes or active rectifiers.
Generators which may also be used for starting the internal combustion engine are known (“starter generator”, RSG). Such a starter generator is typically only operated as a motor at very low speeds, since the torque which may be generated decreases rapidly in the direction of increasing speeds.
Corresponding generators are known, for example, from the field of hybrid vehicles. The goal in this case is to assist the internal combustion engine at low speeds, at which it does not yet deliver its full torque (boost operation, turbo lag compensation).
To be able to provide the required power, such drives are typically operated at a voltage (for example, 48 V) which is significantly higher than the vehicle electrical system voltage (for example, 12 V) provided as standard in the vehicle. Vehicle electrical systems which include two or more voltage levels (for example, 12 V and 48 V) for this and other reasons are also referred to as “two-voltage vehicle electrical systems” or “multi-voltage vehicle electrical systems.”
Terms such as “electric machine,” (electric) drive,” and “electric motor” are used synonymously hereafter. In each case, these are electric machines which may be operated at least sometimes as motors, optionally also as generators, and whose stator winding is commutated via a rectifier using currents (phase currents) according to a predefined activation pattern. A “stator winding” is understood as a system made up of multiple electrical coils. Such a stator winding is designed in the electric machine considered in the present case as a multiphase winding. In this multiphase winding, electrical coils are interconnected individually or in groups in the form of a star or triangle, for example. The number of the coils or also the number of multiple groups of identically connected coils corresponds to the phase number of the electric machine.
From a thermal aspect, the mentioned low speeds and in particular the standstill of the electric machine are particularly critical if the stator winding is further energized in this case. Since the phase currents only change slowly at low speeds and therefore possibly will still be at high levels for a longer time and even have constant values at a standstill, destruction of the activation electronics due to overheating may occur here in a short time.
However, it must be ensured at the same time that the electric machine may also generate a preferably high torque rapidly from a standstill, for example, to implement an electrical startup of the vehicle. A complete interruption of the current feed is therefore not desirable.
The present invention will provide a remedy here and improve the activation of a corresponding electric machine, in particular at a standstill.